CH. XVII.] 



THE NEURON THEORY 



199 



841 



s.c; 



may be taken as follows : Suppose two trees standing side by side ; 



their stems will represent the axis cylinders; their branches the 



dendrons. If the trees are close together the 



branches of one will intermingle with those of 



the other : there is no actual branch from the 



one which becomes continuous with any branch 



of the other; but yet if the stem of one is 



vigorously shaken, the close intermixture of 



the branches will affect the other so that it also 



moves. 



Another very important general idea which 

 we must next get hold of, is that a nervous 

 impulse does not necessarily travel along the 

 same nerve-fibre all the way, but there is what 

 we may term a system of relays. The nervous 

 system is very often compared to a telegraphic 

 system throughout a country. The telegraph 

 offices represent the nerve-centres, the afferent 

 nerve-fibres correspond to the wires that carry 

 the messages to the central offices, and the 

 efferent nerve-fibres are represented by the wires 

 that convey messages from the central offices to 

 more or less distant parts of the country. This 

 illustration will serve us very well for our 

 present purpose, provided that it is always re- 

 membered that a nervous impulse is not elec- 

 tricity. Suppose, now, one wishes to send a 

 message from the metropolis, which will repre- 

 sent the brain, to a distant house, say in the 

 Highlands of Scotland. There is no wire straight 

 from London to that house, but the message 

 ultimately reaches the house ; one wire takes 

 the message to Edinburgh ; another wire carries 

 it on to the telegraph station in the town 

 nearest to the house in question ; and the last 

 part of the journey is accomplished by a mes- 

 senger on foot or horseback. There are at least 

 two relays on the journey. 



It is just the same with the nervous system. 

 Suppose one wishes to move the arm; the im- 

 pulse starts in the nerve-cells of the brain, but 

 there are no fibres that go straight from the 

 brain to the muscles of the arm. The impulse 

 travels down the spinal cord, by what are called pyramidal fibres, 

 which form synapses with the nerve-cells of the spinal cord, and 



S. 



FIG. 213. Diagram of an ele- 

 ment of the motor path. 

 U.S., upper segment ; 

 L.8., lower segment; 

 C.C., cell of cerebral cor- 

 tex; S.C., cell of spinal 

 cord, in anterior cornu ; 

 M., the muscle ; S., path 

 from sensory nerve-roots. 

 (After Gowers.) 



